Fixes issue1336 - a race condition could occur when forking if the gc

kicked in during the critical section.  solution: disable gc during
that section.  Patch contributed by jpa and updated by me to cover the
race condition still existing what therve from twistedmatrix pointed
out (already seen and fixed in twisted's own subprocess code).

Lib/subprocess.py

@@ -356,6 +356,7 @@ mswindows = (sys.platform == "win32")
 import os
 import types
 import traceback
+import gc
 
 # Exception classes used by this module.
 class CalledProcessError(Exception):
@@ -994,7 +995,17 @@ class Popen(object):
             errpipe_read, errpipe_write = os.pipe()
             self._set_cloexec_flag(errpipe_write)
 
+            gc_was_enabled = gc.isenabled()
+            # Disable gc to avoid bug where gc -> file_dealloc ->
+            # write to stderr -> hang.  http://bugs.python.org/issue1336
+            gc.disable()
+            try:
                 self.pid = os.fork()
+            except:
+                if gc_was_enabled:
+                    gc.enable()
+                raise
+            else:
                 self._child_created = True
                 if self.pid == 0:
                     # Child
@@ -1054,6 +1065,8 @@ class Popen(object):
                     os._exit(255)
 
             # Parent
+            if gc_was_enabled:
+                gc.enable()
             os.close(errpipe_write)
             if p2cread is not None and p2cwrite is not None:
                 os.close(p2cread)